A dual fuel engine is an engine that includes a first fuel source that is utilized as the sole fuel source during certain operating conditions and a second fuel source that is integrated with the first fuel source at other operating conditions. In certain applications, the first fuel source is a diesel fuel and the second fuel source is natural gas. The diesel fuel provides, in some cases, the initial, low load levels of operation and is used for ignition for the natural gas at higher load operations. The substitution of natural gas for diesel fuel improves high load performance and emissions reduction, particularly when the engine is employed at locations where natural gas is abundant or available at low cost.
When the engine is operating in dual fuel mode, natural gas is introduced into the intake system. The air-to-natural gas mixture from the intake is drawn into the cylinder, just as it would be in a spark-ignited engine, but typically with a leaner air-to-fuel ratio. Near the end of the compression stroke, diesel fuel is injected, just as it would be in a traditional diesel engine. The diesel fuel ignites, and the diesel combustion causes the natural gas to burn. The dual fuel engine combusts a mixture of air and fuel in the cylinders to produce drive torque. A dual fuel engine can operate either entirely on diesel fuel or on the substitution mixture of diesel and natural gas, but generally cannot operate on natural gas alone except where auxiliary spark equipment is provided to the cylinder.
Dual fuel engines encounter difficulties during operation in response to a load shedding event. While diesel fueling can be readily cut off from the cylinders, the natural gas cannot be cut off as quickly. As a result, uncombusted fuel can enter the exhaust, resulting in possible undesired combustion of the fuel in the exhaust system and/or aftertreatment system. Thus, there remains a need for additional improvements in systems and methods for providing and controlling fuelling in dual fuel engines.